This invention relates generally to plasma processing systems and, more particularly, relates to inductively-coupled plasma processing systems for cleaning a substrate surface before depositing a coating material.
Plasmas are widely used in materials processing for treating the surfaces of substrates, such as semiconductor wafers and flat panel displays, prior to a processing step. In particular, plasmas are used to remove native oxide layers and other contaminants from the substrate surface in preparation for a subsequent deposition of a film of coating material, such as a metallization layer, onto the surface. If the contaminants were not removed by a pre-deposition cleaning process, the physical characteristics, such as the electrical and mechanical properties, of the interface between the layer of coating material and the substrate would be adversely affected.
A conventional approach for removing contaminants is to expose the substrate surface to a plasma in a plasma cleaning step before depositing the film of coating material. The plasma cleaning step may rely on a plasma as a source of reactive species that chemically react with the contamination to form volatile or quasi-volatile products. For example, oxidation can be cleaned from copper metallization on a substrate surface using a hydrogen-containing plasma that chemically reduces the oxide to form volatile etch products. Alternatively, the plasma cleaning step may rely on sputtering due to ion bombardment for cleaning contamination from the substrate surface. For example, oxidation can be removed from aluminum metallization by bombarding the substrate surface with energetic ions from a plasma generated from a noble process gas. Other plasma cleaning steps combine chemical and physical mechanisms for removing contamination from the substrate surface by bombarding the substrate surface with energetic chemically-active plasma species. Preferably, the plasma cleaning removes contaminants from the surface without causing damage or altering the properties of any existing film residing on the surface.
Conventional plasma processing systems designed for plasma cleaning or plasma etching have a vacuum chamber that incorporates a window formed of a dielectric material, such as quartz, and an antenna adjacent the non-vacuum side of the window. Radiofrequency (RF) energy is coupled from the antenna through the dielectric material of the window to the plasma. In certain conventional plasma processing systems, the dielectric window is a bell jar of dielectric material which is sealed to a metal chamber base to define a vacuum chamber. In other conventional plasma processing systems, the dielectric window is a cylindrical or planar structural wall section of dielectric material incorporated into the chamber wall of the vacuum chamber.
Conventional plasma processing systems that utilize a plasma for cleaning substrate surfaces have certain significant disadvantages. In particular, contaminant material sputtered from the substrate surface tends to travel in line-of-sight paths from the substrate toward the interior surfaces of the vacuum chamber. The sputtered contaminant material accumulates, possibly along with chemically-active species originating from the plasma and volatile or quasi-volatile species removed from the substrate surface, as a residue or buildup on interior surfaces, such as the vacuum-side surface of the dielectric window. The residues generated by processing can flake and break off as small particles that are a source of particulate matter detrimental to the fabrication of semiconductor devices. In particular, the residue has a particularly poor adhesion to the surface of the dielectric window. When the plasma is extinguished, the particulate matter can be electrostatically attracted to the substrate. Alternatively, small particles of particulate matter can grow in size while suspended within the plasma and, when the plasma is extinguished, fall under the influence of gravity to the substrate. Such particulate matter may locally compromise the quality of the coating material subsequently deposited on the substrate surface and, thereby, act as defects that reduce device yield.
The accumulation of metal on the dielectric window is a particularly acute problem if the substrates to be sputter cleaned have a significant surface coverage of metal. In particular, the sputter cleaning of metal-covered surfaces produces relatively large accumulations of contaminant residue which serves as a potential source of particulate matter. Moreover, sputtered metal that accumulates on the vacuum-side surface of the dielectric window can affect the operation of the plasma processing system. If the residue is conductive, currents circulating in the buildup tend to reduce the effectiveness of the coupling of RF energy from the antenna to the plasma. Even if the accumulated metal is highly resistive and not limiting of the coupling of the RF energy, the metal residue on the dielectric window can still inhibit plasma ignition and decrease the efficiency of radiofrequency power transmission through the window.
To reduce the occurrence of particulate matter and to maintain efficient coupling of RF energy, the vacuum-side of the dielectric window must be periodically cleaned by chemical and/or abrasive techniques to remove the accumulated residue. Cumulative damage from successive cleanings gradually degrades the mechanical properties of the dielectric material forming the window. As a result, the service life of the dielectric window is reduced and the likelihood of a premature catastrophic failure is enhanced. Typically, the dielectric window is removed from service when the mechanical properties are degraded such that the window can no longer safely support the load applied by atmospheric pressure to the non-vacuum side of the window.
Electron temperature and plasma uniformity are important factors that are balanced such that the plasma distribution is relatively uniform at an operating pressure where the electron temperature is not excessive. Non-uniform plasma densities and excessive electron temperatures can damage the substrates. Asymmetries in the plasma density distribution can result in non-uniform etching or cleaning of the substrates. Although the electron temperature can be reduced by raising the operating pressure of the process gas in the vacuum chamber, the increased operating pressure frequently reduces the uniformity of the plasma density distribution.
The geometry of the vacuum chamber system is another important factor in determining plasma density and plasma uniformity. Ultimately, the processing uniformity over the surface area of the substrate is directly related to the uniformity of the plasma adjacent to the exposed surface of the substrate. Furthermore, in conventional plasma processing systems that utilize chemical activity during treatment, the concentration of chemically-active species from the plasma is depleted near the substrate center and increased near the substrate""s peripheral edge due to gas flow inhomogeneities. This nonuniformity enhances treatment rates at the substrate periphery than at the substrate center, resulting in high center-to-edge nonuniformity. The asymmetrical treatment due to non-uniform plasmas and inhomogeneous concentrations of chemically-active species from the plasma is compounded for large-diameter substrates, such as 300 mm silicon wafers.
Conventional plasma processing systems must be optimized to accommodate large-diameter wafers. For example, to provide a uniformly-distributed plasma near the substrate, the footprint of the antenna and the associated dielectric window must be increased and the plasma source-to-substrate separation distance must be increased. To achieve an acceptable plasma uniformity with a reasonable electron temperature in a large-diameter substrate plasma processing system, the cost of manufacturing the dielectric window increases significantly.
Dielectric windows for large-diameter substrate plasma processing systems face certain technological challenges when the processing system is optimized. As mentioned above, atmospheric pressure applies a significant force distributed over the area of the non-vacuum or ambient side of the dielectric window. Accordingly, the dielectric window must have a thickness that can withstand the applied force or load due to the pressure differential existing between the interior and exterior of the vacuum chamber. For example, the thickness of a 35-centimeter (cm) diameter planar dielectric window, which might be appropriate for processing a 300 mm wafer, must be able to withstand an applied force of about 2200 pounds (lbs) arising from standard atmospheric pressure of 14.7 pounds per square inch acting over the area of the window.
Ceramic dielectric materials are generally brittle and prone to failure under an applied load. The ceramic material forming the dielectric window must be rather thick to withstand the force applied by atmospheric pressure. Thick dielectric windows reduce the efficiency of the coupling with the plasma due to attenuation of RF power in traversing the breadth of the window. Thus, the transfer of RF energy from the antenna to the plasma is inefficient in plasma processing systems having conventional planar dielectric windows. To compensate for the inefficiency, the RF power source must be operated at significantly elevated power levels to increase the RF current delivered to the antenna and provide an acceptable RF power to the plasma. However, the passage of an increased RF current through the antenna increases the Joule heating, which may be adverse to the performance and operation of the plasma processing system if the heat energy is not adequately dissipated.
Conventional plasma processing systems require a uniform distribution of process gas to achieve a uniform plasma distribution. The uniformity of the plasma distribution is adversely affected by asymmetrical distribution of the process gas into and within the vacuum chamber. Generally, gas distribution is affected by both the flow of process gas into the vacuum chamber and the pumping of process gas out of the vacuum chamber. In particular, the distribution of the plasma density is highly sensitive to the uniformity of the gas flow. Furthermore, the uniformity of the distribution of various plasma species can be affected by the distribution of the process gas.
As a result of the above and other considerations and problems, there remains a need for an plasma processing system that efficiently couples radiofrequency energy to the plasma and that can provide a plasma with spatial uniformity for uniformly etching or cleaning the exposed surfaces of substrates, and in particular, the exposed surfaces of large-diameter substrates.
According to the principles of the present invention, a plasma processing system has a vacuum chamber with a chamber wall which surrounds a vacuum processing space. A gas inlet is provided in the chamber wall for introducing a process gas into the vacuum processing space. A substrate support is positioned within the vacuum processing space and is adapted to receive and support a substrate. The plasma processing system is provided with a support member positioned in an opening in the chamber wall. A frustoconical panel of the support member, which is configured to allow radiofrequency (RF) energy to enter the vacuum processing space, mechanically supports a frustoconical section of a dielectric window. An antenna is positioned adjacent to the frustoconical section of the dielectric window and is electrically connected to an RF power supply. The antenna is capable of providing RF energy for transmission through the dielectric window to the vacuum processing space for forming a plasma from the process gas therein.
In one aspect of the present invention, the dielectric window may be formed from a dielectric material such as aluminum oxide, aluminum nitride, silicon nitride, borosilicate glass or quartz. Alternatively, the dielectric material of choice may be a polymer or, more particularly, the polymer may be a polytetrafluoroethylene (PTFE) or a filled PTFE.
In another aspect of the present invention, the frustoconical panel of the deposition baffle extends upwardly with an included angle greater than or equal to 25xc2x0. Preferably, the included angle is about 60xc2x0.
In certain embodiments of the present invention, a plasma processing system further includes a gas source positioned above the substrate support, integral with the support member, and is in fluid communication with the gas inlet. The gas source supplies a flow of the process gas at multiple locations into the vacuum processing space, wherein the process gas is energized by the RF energy to form a plasma. The gas source may comprise an internal gas passageway disposed within the deposition baffle and a plurality of gas ports provided in the internal gas passageway for emitting the flow of the process gas above the substrate support. Alternatively, the gas source may comprise a gas distribution plate having a gas plenum and a plurality of gas ports therein for emitting the flow of the process gases above the substrate support. In yet another alternative, the gas source comprises a gas distribution ring having a plurality of gas ports therein for emitting the flow of the process gases into the vacuum processing space above the substrate support.
According to the present invention, the frustoconical section of the dielectric window is mechanically supported by a frustoconical panel of the deposition baffle so that the thickness of dielectric material can be reduced and still withstand the force applied by atmospheric pressure acting on the window. As a result of the reduction in thickness, the transfer of RF energy from the antenna through the dielectric window to the plasma is more efficient. In addition, the cost to manufacture the dielectric window is significantly decreased by the reduction in the required thickness of the dielectric material. Moreover, the support member of the present invention includes slots configured to shield the dielectric window from the buildup of sputtered etch products, which could otherwise flake and break to create particulate matter or could reduced the efficiency of the transfer of RF energy. The use of one or more of a gas distribution plate, a gas ring, or gas passageways in the support member significantly improves the spatial distribution of the flow of process gas into the vacuum chamber and, thereby, enhances the uniformity and symmetry of the plasma density. The frustoconical shape of the plasma source significantly reduces or eliminates gas recirculation zones to lessen the generation of particulate matter. Forming the dielectric window of PTFE or a filled PTFE significantly decreases the cost of the window. Further, because PTFE is significantly less brittle than ceramic dielectric materials, the likelihood of a catastrophic window failure is significantly reduced.